Due to an output filter capacitor, a substantial amount of harmonic distortion exists in an input current of a conventional bridge rectifying circuit. The harmonic distortion leads a low power factor and severe current total harmonic distortion that undesirably affect an input power. Therefore, power factor correction needs to be performed on a conventional bridge rectifying circuit.
Based on whether active switch elements are included, power factor correction circuits are categorized into two types—a passive type and an active type.
An active power factor correction circuit yields a power factor of above 0.99 and has current total harmonic distortion of less than 10%. An active power factor correction circuit further has a wide-range input voltage, a stable output voltage and small magnetic components as well as being unaffected by output power change.
In order to reduce electromagnetic interference of high-frequency components, a switching frequency of an active switch in a low-frequency power factor correction circuit is twice of the market-electricity frequency. Through changes in a cut-off time and a conduction time of a power switch, a low-frequency power factor correction circuit renders a quite satisfactory power factor. A low-frequency power factor correction circuit is advantaged by having high efficiency and simple control, requiring no high-speed power elements, and being high-frequency electromagnetic interference-free. For inverter-based household appliances, a low-frequency power factor correction circuit effectively improves the power factor and reduces current total harmonic distortion.
Although having a power factor lower than that of an active power factor correction circuit, a passive power factor correction circuit is still capable of increasing the power factor of a circuit to 0.7 to 0.9 and reducing current total harmonic distortion to below 50%. In addition, a passive power factor correction circuit, having advantages of requiring no active switch elements as well as being simple in circuit structure, low in cost and electromagnetic interference-free as it does not contain any active switch elements, is prevalent in medium-power and small-power electronic apparatuses.